System and method for obtaining control of a logical unit number

ABSTRACT

A method, computer program product, and computing system for receiving a Mode Select command concerning a logical unit number (LUN) from a first host, wherein the Mode Select command defines control information and host identifier information concerning the first host, and the LUN is currently being controlled by a second host. The Mode Select command is processed to determine if the control information and host identifier information included within the Mode Select command signifies an intent by the first host to seize control of the LUN from the second host. If the control information and host identifier information signifies an intent to seize control of the LUN from the second host, the control information and host identifier information included within the Mode Select command is written to a buffer associated with the LUN, wherein the buffer includes a control field and a globally unique identifier (GUID) field.

TECHNICAL FIELD

This disclosure relates to cache memory systems and, more particularly,to systems and methods for improving the performance of cache memorysystems.

BACKGROUND

Storing and safeguarding electronic content is of paramount importancein modern business. Accordingly, various systems may be employed toprotect such electronic content.

The use of solid-state storage devices is increasing in popularity. Asolid state storage device is a content storage device that usessolid-state memory to store persistent content. A solid-state storagedevice may emulate (and therefore replace) a conventional hard diskdrive. Additionally/alternatively, a solid state storage device may beused within a cache memory system. With no moving parts, a solid-statestorage device largely eliminates (or greatly reduces) seek time,latency and other electromechanical delays and failures associated witha conventional hard disk drive.

SUMMARY OF DISCLOSURE

In a first implementation, a computer-implemented method includesreceiving a Mode Select command concerning a LUN from a first host,wherein the Mode Select command defines control information and hostidentifier information concerning the first host, and the LUN iscurrently being controlled by a second host. The Mode Select command isprocessed to determine if the control information and host identifierinformation included within the Mode Select command signifies an intentby the first host to seize control of the LUN from the second host. Ifthe control information and host identifier information signifies anintent to seize control of the LUN from the second host, the controlinformation and host identifier information included within the ModeSelect command is written to a buffer associated with the LUN, whereinthe buffer includes a control field and a GUID field.

One or more of the following features may be included. Writing thecontrol information and host identifier information included within theMode Select command may include writing a zero to the control fieldwithin the buffer.

After the expiry of a defined verification period, the buffer associatedwith the LUN may be read to determine if the second host is stillviable. The second host may be allowed to modify the buffer during theverification period to indicate viability. If the second host is stillviable, the attempt by the first host to seize control of the LUN fromthe second host may be aborted. If the second host is not still viable,control of the LUN may be transferred from the second host to the firsthost. The first host and the second host may be application servers.

In another implementation, a computer program product resides on acomputer readable medium that has a plurality of instructions stored onit. When executed by a processor, the instructions cause the processorto perform operations including receiving a Mode Select commandconcerning a LUN from a first host, wherein the Mode Select commanddefines control information and host identifier information concerningthe first host, and the LUN is currently being controlled by a secondhost. The Mode Select command is processed to determine if the controlinformation and host identifier information included within the ModeSelect command signifies an intent by the first host to seize control ofthe LUN from the second host. If the control information and hostidentifier information signifies an intent to seize control of the LUNfrom the second host, the control information and host identifierinformation included within the Mode Select command is written to abuffer associated with the LUN, wherein the buffer includes a controlfield and a GUID field.

One or more of the following features may be included. Writing thecontrol information and host identifier information included within theMode Select command may include writing a zero to the control fieldwithin the buffer.

After the expiry of a defined verification period, the buffer associatedwith the LUN may be read to determine if the second host is stillviable. The second host may be allowed to modify the buffer during theverification period to indicate viability. If the second host is stillviable, the attempt by the first host to seize control of the LUN fromthe second host may be aborted. If the second host is not still viable,control of the LUN may be transferred from the second host to the firsthost. The first host and the second host may be application servers.

In another implementation, a computing system includes at least oneprocessor and at least one memory architecture coupled with the at leastone processor, wherein the computing system is configured to performoperations including receiving a Mode Select command concerning a LUNfrom a first host, wherein the Mode Select command defines controlinformation and host identifier information concerning the first host,and the LUN is currently being controlled by a second host. The ModeSelect command is processed to determine if the control information andhost identifier information included within the Mode Select commandsignifies an intent by the first host to seize control of the LUN fromthe second host. If the control information and host identifierinformation signifies an intent to seize control of the LUN from thesecond host, the control information and host identifier informationincluded within the Mode Select command is written to a bufferassociated with the LUN, wherein the buffer includes a control field anda GUID field.

One or more of the following features may be included. Writing thecontrol information and host identifier information included within theMode Select command may include writing a zero to the control fieldwithin the buffer.

After the expiry of a defined verification period, the buffer associatedwith the LUN may be read to determine if the second host is stillviable. The second host may be allowed to modify the buffer during theverification period to indicate viability. If the second host is stillviable, the attempt by the first host to seize control of the LUN fromthe second host may be aborted. If the second host is not still viable,control of the LUN may be transferred from the second host to the firsthost. The first host and the second host may be application servers.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a storage system and a data cachingprocess coupled to a distributed computing network;

FIG. 2 is a diagrammatic view of the storage system of FIG. 1;

FIG. 3 is a diagrammatic view of a data write request for use with thedata caching process of FIG. 1;

FIG. 4 is a diagrammatic view of a data read request for use with thedata caching process of FIG. 1;

FIG. 5 is a diagrammatic view of a content directory for use with thedata caching process of FIG. 1;

FIG. 6 is a first flow chart of the data caching process of FIG. 1;

FIG. 7 is a second flow chart of the data caching process of FIG. 1;

FIG. 8 is a third flow chart of the data caching process of FIG. 1; and

FIG. 9 is a fourth flow chart of the data caching process of FIG. 1.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS General Information

As will be appreciated by one skilled in the art, the present disclosuremay be embodied as a method, system, or computer program product.Accordingly, the present disclosure may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,the present disclosure may take the form of a computer program producton a computer-usable storage medium having computer-usable program codeembodied in the medium.

Any suitable computer usable or computer readable medium may beutilized. The computer-usable or computer-readable medium may be, forexample but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. More specific examples (a non-exhaustive list) ofthe computer-readable medium would include the following: an electricalconnection having one or more wires, a portable computer diskette, ahard disk, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), anoptical fiber, a portable compact disc read-only memory (CD-ROM), anoptical storage device, a transmission media such as those supportingthe Internet or an intranet, or a magnetic storage device. Note that thecomputer-usable or computer-readable medium could even be paper oranother suitable medium upon which the program is printed, as theprogram can be electronically captured, via, for instance, opticalscanning of the paper or other medium, then compiled, interpreted, orotherwise processed in a suitable manner, if necessary, and then storedin a computer memory. In the context of this document, a computer-usableor computer-readable medium may be any medium that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer-usable medium may include a propagated data signal with thecomputer-usable program code embodied therewith, either in baseband oras part of a carrier wave. The computer usable program code may betransmitted using any appropriate medium, including but not limited tothe Internet, wireline, optical fiber cable, RF, etc.

Computer program code for carrying out operations of the presentdisclosure may be written in an object oriented programming languagesuch as Java, Smalltalk, C++ or the like. However, the computer programcode for carrying out operations of the present disclosure may also bewritten in conventional procedural programming languages, such as the“C” programming language or similar programming languages. The programcode may execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

The present disclosure is described below with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the disclosure. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

System Overview:

Referring to FIG. 1, there is shown data caching process 10 that mayreside on and may be executed by storage system 12, which may beconnected to network 14 (e.g., the Internet or a local area network).Examples of storage system 12 may include, but are not limited to: aNetwork Attached Storage (NAS) system, a Storage Area Network (SAN), apersonal computer with a memory system, a server computer with a memorysystem, and a cloud-based device with a memory system.

As is known in the art, a SAN may include one or more of a personalcomputer, a server computer, a series of server computers, a minicomputer, a mainframe computer, a RAID device and a NAS system. Thevarious components of storage system 12 may execute one or moreoperating systems, examples of which may include but are not limited to:Microsoft Windows XP Server™; Novell Netware™; Redhat Linux™, Unix, or acustom operating system, for example.

The instruction sets and subroutines of data caching process 10, whichmay be stored on storage device 16 included within storage system 12,may be executed by one or more processors (not shown) and one or morememory architectures (not shown) included within storage system 12.Storage device 16 may include but is not limited to: a hard disk drive;a tape drive; an optical drive; a RAID device; a random access memory(RAM); a read-only memory (ROM); and all forms of flash memory storagedevices.

Network 14 may be connected to one or more secondary networks (e.g.,network 18), examples of which may include but are not limited to: alocal area network; a wide area network; or an intranet, for example.

Various data requests (e.g. data request 20) may be sent from clientapplications 22, 24, 26, 28 to storage system 12. Examples of datarequest 20 may include but are not limited to data write requests (i.e.a request that content be written to storage system 12) and data readrequests (i.e. a request that content be read from storage system 12).

The instruction sets and subroutines of client applications 22, 24, 26,28, which may be stored on storage devices 30, 32, 34, 36 (respectively)coupled to client electronic devices 38, 40, 42, 44 (respectively), maybe executed by one or more processors (not shown) and one or more memoryarchitectures (not shown) incorporated into client electronic devices38, 40, 42, 44 (respectively). Storage devices 30, 32, 34, 36 mayinclude but are not limited to: hard disk drives; tape drives; opticaldrives; RAID devices; random access memories (RAM); read-only memories(ROM), and all forms of flash memory storage devices. Examples of clientelectronic devices 38, 40, 42, 44 may include, but are not limited to,personal computer 38, laptop computer 40, personal digital assistant 42,notebook computer 44, a server (not shown), a data-enabled, cellulartelephone (not shown), and a dedicated network device (not shown).

Client electronic devices 38, 40, 42, 44 may each execute an operatingsystem, examples of which may include but are not limited to MicrosoftWindows™, Microsoft Windows CE™, Redhat Linux™, or a custom operatingsystem.

Users 46, 48, 50, 52 may access storage system 12 directly throughnetwork 14 or through secondary network 18. Further, storage system 12may be connected to network 14 through secondary network 18, asillustrated with link line 54.

The various client electronic devices may be directly or indirectlycoupled to network 14 (or network 18). For example, personal computer 38is shown directly coupled to network 14 via a hardwired networkconnection. Further, notebook computer 44 is shown directly coupled tonetwork 18 via a hardwired network connection. Laptop computer 40 isshown wirelessly coupled to network 14 via wireless communicationchannel 56 established between laptop computer 40 and wireless accesspoint (i.e., WAP) 58, which is shown directly coupled to network 14. WAP58 may be, for example, an IEEE 802.11a, 802.11b, 802.11g, 802.11n,Wi-Fi, and/or Bluetooth device that is capable of establishing wirelesscommunication channel 56 between laptop computer 40 and WAP 58. Personaldigital assistant 42 is shown wirelessly coupled to network 14 viawireless communication channel 60 established between personal digitalassistant 42 and cellular network/bridge 62, which is shown directlycoupled to network 14.

As is known in the art, all of the IEEE 802.11x specifications may useEthernet protocol and carrier sense multiple access with collisionavoidance (i.e., CSMA/CA) for path sharing. The various 802.11xspecifications may use phase-shift keying (i.e., PSK) modulation orcomplementary code keying (i.e., CCK) modulation, for example. As isknown in the art, Bluetooth is a telecommunications industryspecification that allows e.g., mobile phones, computers, and personaldigital assistants to be interconnected using a short-range wirelessconnection.

The Data Caching Process:

For the following discussion, client application 22 is going to bedescribed for illustrative purposes. However, this is not intended to bea limitation of this disclosure, as other client applications (e.g.,client applications 24, 26, 28) may be equally utilized.

For illustrative purposes, storage system 12 will be described as beinga network-based storage system that includes a plurality ofelectro-mechanical backend storage devices. However, this is forillustrative purposes only and is not intended to be a limitation ofthis disclosure, as other configurations are possible and are consideredto be within the scope of this disclosure. For example and as discussedabove, storage system 12 may be a personal computer that includes asingle electro-mechanical storage device.

Referring also to FIG. 2, storage system 12 may include a servercomputer/controller (e.g. server computer/controller 100), and aplurality of storage targets T_(1-n) (e.g. storage targets 102, 104,106, 108). Storage targets 102, 104, 106, 108 may be configured toprovide various levels of performance and/or high availability. Forexample, one or more of storage targets 102, 104, 106, 108 may beconfigured as a RAID 0 array, in which data is striped across storagetargets. By striping data across a plurality of storage targets,improved performance may be realized. However, RAID 0 arrays do notprovide a level of high availability. Accordingly, one or more ofstorage targets 102, 104, 106, 108 may be configured as a RAID 1 array,in which data is mirrored between storage targets. By mirroring databetween storage targets, a level of high availability is achieved asmultiple copies of the data are stored within storage system 12.

While storage targets 102, 104, 106, 108 are discussed above as beingconfigured in a RAID 0 or RAID 1 array, this is for illustrativepurposes only and is not intended to be a limitation of this disclosure,as other configurations are possible. For example, storage targets 102,104, 106, 108 may be configured as a RAID 3, RAID 4, RAID 5 or RAID 6array.

While in this particular example, storage system 12 is shown to includefour storage targets (e.g. storage targets 102, 104, 106, 108), this isfor illustrative purposes only and is not intended to be a limitation ofthis disclosure. Specifically, the actual number of storage targets maybe increased or decreased depending upon e.g. the level ofredundancy/performance/capacity required.

Storage system 12 may also include one or more coded targets 110. As isknown in the art, a coded target may be used to store coded data thatmay allow for the regeneration of data lost/corrupted on one or more ofstorage targets 102, 104, 106, 108. An example of such a coded targetmay include but is not limited to a hard disk drive that is used tostore parity data within a RAID array.

While in this particular example, storage system 12 is shown to includeone coded target (e.g., coded target 110), this is for illustrativepurposes only and is not intended to be a limitation of this disclosure.Specifically, the actual number of coded targets may be increased ordecreased depending upon e.g. the level ofredundancy/performance/capacity required.

Examples of storage targets 102, 104, 106, 108 and coded target 110 mayinclude one or more electro-mechanical hard disk drives, wherein acombination of storage targets 102, 104, 106, 108 and coded target 110may form non-volatile, electro-mechanical memory system 112.

The manner in which storage system 12 is implemented may vary dependingupon e.g. the level of redundancy/performance/capacity required. Forexample, storage system 12 may be a RAID device in which servercomputer/controller 100 is a RAID controller card and storage targets102, 104, 106, 108 and/or coded target 110 are individual“hot-swappable” hard disk drives. An example of such a RAID device mayinclude but is not limited to an NAS device. Alternatively, storagesystem 12 may be configured as a SAN, in which servercomputer/controller 100 may be e.g., a server computer and each ofstorage targets 102, 104, 106, 108 and/or coded target 110 may be a RAIDdevice and/or computer-based hard disk drive. Further still, one or moreof storage targets 102, 104, 106, 108 and/or coded target 110 may be aSAN.

In the event that storage system 12 is configured as a SAN, the variouscomponents of storage system 12 (e.g. server computer/controller 100,storage targets 102, 104, 106, 108, and coded target 110) may be coupledusing network infrastructure 114, examples of which may include but arenot limited to an Ethernet (e.g., Layer 2 or Layer 3) network, a fiberchannel network, an InfiniBand network, or any other circuitswitched/packet switched network.

Storage system 12 may execute all or a portion of data caching process10. The instruction sets and subroutines of data caching process 10,which may be stored on a storage device (e.g., storage device 16)coupled to server computer/controller 100, may be executed by one ormore processors (not shown) and one or more memory architectures (notshown) included within server computer/controller 100. Storage device 16may include but is not limited to: a hard disk drive; a tape drive; anoptical drive; a RAID device; a random access memory (RAM); a read-onlymemory (ROM); and all forms of flash memory storage devices.

As discussed above, various data requests (e.g. data request 20) may begenerated. For example, these data requests may be sent from clientapplications 22, 24, 26, 28 to storage system 12.Additionally/alternatively and when server computer/controller 100 isconfigured as an application server, these data requests may beinternally generated within server computer/controller 100. Examples ofdata request 20 may include but are not limited to data write request116 (i.e. a request that content 118 be written to storage system 12)and data read request 120 (i.e. a request that content 118 be read fromstorage system 12).

Server computer/controller 100 may include input-output logic 122 (e.g.,a network interface card or a Host Bus Adaptor (HBA)), processing logic124, and first cache system 126. Examples of first cache system 126 mayinclude but are not limited to a volatile, solid-state, cache memorysystem (e.g., a dynamic RAM cache memory system) and/or a non-volatile,solid-state, cache memory system (e.g., a flash-based, cache memorysystem).

During operation of server computer/controller 100, content 118 to bewritten to storage system 12 may be received by input-output logic 122(e.g. from network 14 and/or network 18) and processed by processinglogic 124. Additionally/alternatively and when servercomputer/controller 100 is configured as an application server, content118 to be written to storage system 12 may be internally generated byserver computer/controller 100. As will be discussed below in greaterdetail, processing logic 124 may initially store content 118 withinfirst cache system 126.

Depending on the manner in which first cache system 126 is configured,processing logic 124 may immediately write content 118 to second cachesystem 128/non-volatile, electro-mechanical memory system 112 (if firstcache system 126 is configured as a write-through cache) or maysubsequently write content 118 to second cache system 128/non-volatile,electro-mechanical memory system 112 (if first cache system 126 isconfigured as a write-back cache). Additionally and in certainconfigurations, processing logic 124 may calculate and store coded dataon coded target 110 (included within non-volatile, electromechanicalmemory system 112) that may allow for the regeneration of datalost/corrupted on one or more of storage targets 102, 104, 106, 108. Forexample, if processing logic 124 was included within a RAID controllercard or a NAS/SAN controller, processing logic 124 may calculate andstore coded data on coded target 110. However, if processing logic 124was included within e.g., an application server, data array 130 maycalculate and store coded data on coded target 110.

Examples of second cache system 128 may include but are not limited to avolatile, solid-state, cache memory system (e.g., a dynamic RAM cachememory system) and/or a non-volatile, solid-state, cache memory system(e.g., a flash-based, cache memory system).

The combination of second cache system 128 and non-volatile,electromechanical memory system 112 may form data array 130, whereinfirst cache system 126 may be sized so that the number of times thatdata array 130 is accessed may be reduced. Accordingly, by sizing firstcache system 126 so that first cache system 126 retains a quantity ofdata sufficient to satisfy a significant quantity of data requests(e.g., data request 20), the overall performance of storage system 12may be enhanced. As will be described below in greater detail, firstcache system 126 may be a content-aware cache system.

Further, second cache system 128 within data array 130 may be sized sothat the number of times that non-volatile, electromechanical memorysystem 112 is accessed may be reduced. Accordingly, by sizing secondcache system 128 so that second cache system 128 retains a quantity ofdata sufficient to satisfy a significant quantity of data requests(e.g., data request 20), the overall performance of storage system 12may be enhanced. As will be described below in greater detail, secondcache system 128 may be a content-aware cache system.

As discussed above, the instruction sets and subroutines of data cachingprocess 10, which may be stored on storage device 16 included withinstorage system 12, may be executed by one or more processors (not shown)and one or more memory architectures (not shown) included within storagesystem 12. Accordingly, in addition to being executed on servercomputer/controller 100, some or all of the instruction sets andsubroutines of data caching process 10 may be executed by one or moreprocessors (not shown) and one or more memory architectures (not shown)included within data array 130.

Referring also to FIGS. 3-4, data request 20 (e.g. data read request 116and/or data write request 120) may be processed by servercomputer/controller 100 to extract pertinent information concerningthese data requests.

When data request 20 is a data write request (e.g., write request 116),write request 116 may include content 118 to be written to data array130. Additionally, write request 116 may include a storage address 200that defines the intended storage location within storage array 130 atwhich content 118 is to be stored. For example, storage address 200 maydefine a particular logical unit within data array 130 (e.g., a LUN orLogical Unit Number) and a particular storage address within thatspecific logical unit (e.g., an LBA or Logical Block Address) forstoring content 118.

Concerning read request 120, these requests do not include any contentto be written to data array 130, as these are read requests and concerncontent to be read from data array 130. Read request 120 may include astorage address 202 that defines the storage location within storagearray 130 from which content is to be retrieved. For example, storageaddress 202 may define a particular logical unit within data array 130(e.g., a LUN or Logical Unit Number) and a particular storage addresswithin that specific logical unit (e.g., an LBA or Logical BlockAddress) for retrieving the content sought from data array 130.

As will be discussed below in greater detail and referring also to FIG.5, data caching process 10 may maintain content directory 250, which maybe used to locate various pieces of content within first cache system126. In one particular embodiment of content directory 250, contentdirectory 250 may include plurality of entries 252, wherein each ofthese entries may identify: data array storage address 200/202 (e.g. alogical storage unit and a storage address at which a specific piece ofpreviously-written content is located within storage array 130); firstcache address 254 (e.g., the location within first cache system 126 atwhich the specific piece of previously-written content is also located),and content identifier 256 for the specific piece of previously-writtencontent. Accordingly, content directory 250 may identify the location ofspecific pieces of content included within first cache system 126 andtheir corresponding pieces of data within data array 130, as well as acontent identifier that uniquely identifies the specific piece ofcontent.

Content identifier 256 may be used in a content-aware caching system andmay, specifically, be a mathematical representation of the specificpiece of previously-written content that may allow e.g. servercomputer/controller 100 to quickly determine whether two pieces ofpreviously-written content are identical, as identical pieces of contentwould have identical content identifiers. In one particular embodiment,content identifier 256 may be a hash function (e.g., a cryptographichash) of the previously-written content. Accordingly, through the use ofa content-aware caching system, duplicate data entries within firstcache system 126 and/or second cache system 128 may be quicklyidentified, avoided, and/or eliminated.

As is known in the art, a hash function is an algorithm/subroutine thatmaps large data sets to smaller data sets. The values returned by a hashfunction are typically called hash values, hash codes, hash sums,checksums or simply hashes. Hash functions are mostly used to acceleratetable lookup or data comparison tasks such as e.g., finding items in adatabase and detecting duplicated or similar records in a large file.

General Read Request Processing:

During operation of server computer/controller 100, data caching process10 may receive read request 120 on first cache system 126, wherein readrequest 120 identifies previously-written content (as defined by storageaddress 202) included within data array 130.

For example, assume that user 46 is using client application 22 toaccess data (i.e. content 132) that is currently being stored on dataarray 130. Accordingly, client application 22 may generate read request120 which, as discussed above, may define a particular logical unitwithin data array 130 (e.g., a LUN or Logical Unit Number) and aparticular storage address within that specific logical unit (e.g., anLBA or Logical Block Address) for retrieving content 132 sought fromdata array 130 by client application 22.

Assume that read request 120 defines LUN0/LBA5 as the location ofcontent 132 within data array 130. Upon receiving read request 120, datacaching process 10 may compare the location of content 132 within dataarray 130 (namely LUN0/LBA5) with each of the plurality of entries 252defined within content directory 250 to determine if a copy of content132 is locally available (i.e., cached) within first cache system 126.If LUN0/LBA5 was defined within content directory 250 (meaning that alocal cached copy of content 132 is present/available within first cachesystem 126), that particular entry would also define a correspondingfirst cache address (e.g. first cache address 254) within first cachesystem 126 at which content 132 would be locally-available andretrievable from the first cache system 126. Conversely, in the eventthat LUN0/LBA5 is not defined within content directory 250 (meaning thata local cached copy of content 132 is not present/available within firstcache system 126), data caching process 10 may need to obtain content132 identified in read request 120 from data array 130.

In this particular example, since LUN0/LBA5 is not defined withincontent directory 250, a local cached copy of content 132 is notpresent/available within first cache system 126 and data caching process10 will be need to obtain content 132 from data array 130.

Once content 132 is obtained by data caching process 10 from data array130, data caching process 10 may store content 132 within first cachesystem 126 and may provide content 132 to client application 22, thussatisfying read request 120. Additionally, content directory 250 may beamended by data caching process 10 to include an entry (e.g., entry 258)that defines the data array storage address 200/202 (e.g. LUN0/LBA5);first cache address 254 (e.g., 111110), and content identifier 256(e.g., ablccba) for content 132.

As discussed above, data array 130 may include second cache system 128.Accordingly, data caching process 10 may execute the above-describedfunctionality with respect to second cache system 128.

General Write Request Processing:

During operation of server computer/controller 100, data caching process10 may receive write request 116 on first cache system 126, whereinwrite request 116 identifies new content (e.g., content 118) to bewritten to data array 130.

For example, assume that user 46 is using client application 22 tocreate content (i.e. content 118) that is to be stored on data array130. Accordingly, client application 22 may generate write request 116which, as discussed above, may define a particular logical unit withindata array 130 (e.g., a LUN or Logical Unit Number) and a particularstorage address within that specific logical unit (e.g., an LBA orLogical Block Address) for storing content 118 within data array 130.

As discussed above and depending on the manner in which first cachesystem 126 is configured, data caching process 10 may immediately writecontent 118 to data array 130 (if first cache system 126 is configuredas a write-through cache) or may subsequently write content 118 to dataarray 130 (if first cache system 126 is configured as a write-backcache).

Assuming that first cache system 126 in this example is configured as awrite-through cache, data caching process 10 may immediately writecontent 118 to LUN0/LBA0 within data array 130 (as defined within writerequest 116). Additionally, data caching process 10 may locally-storecontent 118 within first cache system 126 and may amend contentdirectory 250 to include an entry (e.g., entry 260) that defines thedata array storage address 200/202 (e.g. LUN0/LBA0); first cache address254 (e.g., 001011), and content identifier 256 (e.g., acdfcla) forcontent 118.

As discussed above, data array 130 may include second cache system 128.Accordingly, data caching process 10 may execute the above describedfunctionality with respect to second cache system 128.

Content Aware Caching

As discussed above, content directory 250 may include a contentidentifier 256 that may be used in a content-aware caching system. Atypical example of content identifier 256 may include but is not limitedto a hash function of the content that content identifier 256 isassociated with. Accordingly, through the use of content identifier 256within a content-aware caching system, duplicate data entries withinfirst cache system 126 and/or second cache system 128 may be quicklyidentified, avoided, and/or eliminated.

For example, upon receiving write request 116 and content 118, datacaching process 10 may generate content identifier 256 for content 118.As discussed above, content identifier 256 generated for the content(i.e., content 118) identified within write request 116 may be a hashfunction (e.g., a cryptographic hash) of content 118.

Assume for illustrative purposes that write request 116 includes storageaddress 200 that defines the intended storage location for content 118as LUN0/LBA0.

Accordingly, upon receiving write request 116, data caching process 10may generate content identifier 256 for content 118. Assume forillustrative purposes that data caching process 10 generates a hash ofcontent 118, resulting in the generation of content identifier 256(namely hash value acdfcla).

This newly-generated content identifier 256 (i.e. acdfcla) associatedwith content 118 may be compared to each of the other contentidentifiers (namely abalaby, alazchb, abalabz, alazcha) included withincontent directory 250 for first cache system 126 to determine if thenewly-generated content identifier 256 (i.e. acdfcla) matches any of theother content identifiers (namely abalaby, alazchb, abalabz, alazcha)included within content directory 250.

As discussed above, each entry of the plurality of entries 252 includedwithin content directory 250 is associated with a unique piece ofcontent included within (in this example) first cache system 126.Accordingly, each unique content identifier included within contentdirectory 250 may be associated with a unique piece of content writtento (in this example) first cache system 126.

If, when performing this comparison, data caching process 10 does notidentify a content identifier (i.e., abalaby, alazchb, abalabz, alazcha)within content directory 250 that matches the above-described,newly-generated content identifier (i.e. acdfcla), data caching process10 may write content 118 to (in this example) first cache system 126 andmay provide a copy of content 118 to data array 130 for storage withindata array 130. Additionally, data caching process 10 may modify contentdirectory 250 to include a new entry (i.e., entry 260) that defines thenewly-generated content identifier (i.e. acdfcla), the location ofcontent 118 within (in this example) first cache system 126 (i.e.,001011), and the location of content 118 within data array 130 (i.e.,LUN0/LBA0).

If, when performing this comparison, data caching process 10 identifieda content identifier within content directory 250 that matched theabove-described, newly-generated content identifier (i.e. acdfcla), datacaching process 10 would perform differently.

To illustrate how data caching process 10 would react if it found amatching content identifier, further assume for illustrative purposesthat a second write request (i.e., write request 116′) includes storageaddress 200′ that defines the intended storage location for content 118′as LUN0/LBA2. Accordingly, upon receiving write request 116′, datacaching process 10 may generate content identifier 256 for content 118′.Assume for illustrative purposes that data caching process 10 generatesa hash of content 118′, resulting in the generation of contentidentifier 256 (namely hash value alazcha).

This newly-generated content identifier 256 (i.e. alazcha) associatedwith content 118′ may be compared to each of the other contentidentifiers (namely abalaby, alazchb, abalabz, alazcha) included withincontent directory 250 for (in this example) first cache system 126 todetermine if the newly-generated content identifier 256 (i.e. alazcha)matches any of the other content identifiers (namely abalaby, alazchb,abalabz, alazcha) included within content directory 250.

If, when performing this comparison, data caching process 10 doesidentify a content identifier (namely alazcha) within content directory250 that matches the above-described, newly-generated content identifier(i.e. alazcha), data caching process 10 may perform a couple offunctions.

For example, data caching process 10 may modify the entry (i.e., entry262) within content directory 250 that is associated with the matchingcontent identifier (i.e., alazcha) to include storage address 200′ thatdefines the intended storage location for content 118′ (i.e., LUN0/LBA2within data array 130), thus generating modified entry 262′.Accordingly, modified entry 262′ identifies that the pieces of contentthat are currently stored at LUN4/LBA7 and LUN0/LBA2 within data array130 are identical. Accordingly, a single piece of cached content(located at first cache address 010111 within, in this example, firstcache system 126) may be used as a local cached copy for both pieces ofcontent stored on data array 130.

While the system is described above as modifying entry 262 by adding asecond LUN/LBA designation to generate modified entry 262′, this is forillustrative purposes only and is not intended to be a limitation ofthis disclosure, as other configurations are possible. For example,sub-tables/sub-entries may be utilized to show the manner in whichmultiple LUNs/LBAs are mapped to a single piece of content within, forexample, first cache system 126.

As discussed above, data array 130 may include second cache system 128.Accordingly, data caching process 10 may execute the above-describedcontent aware functionality with respect to second cache system 128.

Managing Access to LUNs

Assume for illustrative purposes that a second servercomputer/controller (e.g. server computer/controller 134) is alsocoupled to data array 130 via network infrastructure 114. Accordingly,each of server computer/controller 100 and server computer/controller134 may gain control and relinquish control of the various logical units(e.g. LUNs) within data array 130. Further, each of servercomputer/controller 100 and server computer/controller 134 may writedata to and read data from various storage addresses (e.g., an LBA orLogical Block Address) within the LUNs included in data array 130.

Accordingly, problems may be realized when e.g. servercomputer/controller 100 and server computer/controller 134 perform tasksthat are in conflict with each other. For illustrative purposes, assumethat a first server computer/controller (e.g. server computer/controller100) writes a piece of content (e.g. content 118) to an address (e.g.LUN0/LBA2) within storage array 130. As discussed above, a copy of thiscontent (e.g. content 118) will also be written to first cache system126. Further assume that a second server computer/controller (e.g.server computer/controller 134) writes a piece of content (e.g. content136) to the same address (e.g. LUN0/LBA2) within storage array 130.Similarly, a copy of this content (e.g. content 136) would also bewritten to a local cache system (e.g., cache system 138) within servercomputer/controller 134.

Accordingly and in this situation, if a read request is receivedconcerning LUN0/LBA2 within data array 130, the data provided to therequester will vary depending upon which server computer/controllerfulfills the request. For example, if the read request was received byserver computer/controller 134, server computer/controller 134 wouldfulfill the read request by providing the locally-cached copy of thedata (which is stored within cache system 138 of servercomputer/controller 134). In this situation, the data provided to therequester would be correct, in that the cached data matches the datastored at LUN0/LBA2 within data array 130.

However, if the read request was received by server computer/controller100, server computer/controller 100 would also fulfill the read requestby providing the locally-cached copy of the data (which is stored withinfirst cache system 126 of server computer/controller 100). Unfortunatelyand in this situation, the data provided to the requester would beincorrect, in that the cached data does not match the data stored atLUN0/LBA2 within data array 130.

Ownership of the various LUNs included within data array 130 may becontrolled via various SCSI commands, namely the Mode Select command andthe Mode Sense command. As is known in the art, SCSI (i.e., SmallComputer System Interface) is a set of standards for physicallyconnecting and transferring data between computers and peripheraldevices. Specifically, the SCSI standards define commands, protocols andinterfaces.

The SCSI Mode Select command may be used to modify device informationcontained in buffers associated with a related device, wherein the SCSIMode Sense command may be used to read the information contained inthese buffers.

In order to control ownership of the various LUNs included within dataarray 130 via the Mode Select command and the Mode Sense command, datacaching process 10 may establish a buffer (e.g., buffer 140) for eachLUN included within data array 130. These buffers are typically locatedwithin data array 130 but need not be located on the physical storagedevice associated with the LUN. This buffer may be written to via theMode Select command and read from via the Mode Sense command.

The Mode Select command and the Mode Sense command issued by datacaching process 10 are different than traditional Mode Select and ModeSense commands, in that data caching process 10 produces an expandedMode Select and Mode Sense command that includes additional data in theform of a vender unique (VU) page that defines a control field thatidentifies the current control status for the LUN (having a possiblevalue of 1 or 0) and a GUID (globally unique identifier) field thatidentifies the host (i.e., the appropriate server computer/controller).The GUID for the host is typically driven by the MAC address for thehost (i.e., the server computer/controller). Accordingly, theabove-described buffers (e.g., buffer 140) need to be sized toaccommodate these expanded Mode Select and Mode Sense commands.

Obtaining Control of a LUN

When the above-described system initially starts up, the control fieldwithin a buffer for a LUN is defined as 0 and the GUID field within abuffer for a LUN is defined as all zeros, thus indicating that no deviceis in control of the related LUN.

Assume for illustrative purposes that the above-described system hasjust started up and LUN 0 (included within data array 130) is not beingcontrolled by any host. During this startup mode, the various hostsincluded within the system may attempt to obtain control of LUN 0. Forexample, server computer/controller 100 and server computer/controller134 may each generate Mode Select commands in an attempt to obtaincontrol of LUN0. For example, server computer/controller 100 maygenerate Mode Select command 142 that defines a control value of 1 and aGUID that identifies server computer/controller 100. Additionally,server computer/controller 134 may generate Mode Select command 144 thatdefines a control value of 1 and a GUID that identifies servercomputer/controller 134.

Data caching process 10 may receive 300 these Mode Select commands(e.g., Mode Select commands 142, 144) concerning this single LUN (e.g.,LUN 0) from each of these potential hosts, namely servercomputer/controller 100 and server computer/controller 134.

As discussed above, each of these Mode Select commands (Mode Selectcommands 142, 144) defines control information and host identifierinformation for its related host, namely server computer/controller 100for Mode Select command 142 and server computer/controller 134 for ModeSelect command 144.

Data caching process 10 may process 302 the Mode Select commandsreceived in a serial fashion and in accordance with the order in whichthe Mode Select commands were received. Accordingly, assume forillustrative purposes that Mode Select command 142 was received by datacaching process 10 before Mode Select command 144. Accordingly, datacaching process 10 may process 302 Mode Select command 142 before ModeSelect command 144. Accordingly, the first Mode Select command received(namely Mode Select command 142) is accepted by data caching process 10and subsequent Mode Select commands received (namely Mode Select command144) are rejected by data caching process 10, thus defining one acceptedMode Select command (namely Mode Select command 142) and one or morerejected Mode Select commands (namely Mode Select command 144).

While in this particular example, two potential hosts are shown (namelyserver computer/controller 100 and server computer/controller 134), thisis for illustrative purposes only and is not intended to be a limitationon this disclosure, as the number of potential hosts may be increased ordecreased accordingly.

For the accepted Mode Select command (namely Mode Select command 142),the control information and host identifier information included withinthe accepted Mode Select command may be written 304 to buffer 140associated with e.g. LUN 0. As discussed above, buffer 140 includes acontrol field and a GUID field. Accordingly, the control informationincluded within Mode Select command 142 may be written 304 to thecontrol field of buffer 140 and the host identifier information includedwithin Mode Select command 142 may be written 304 to the GUID fieldwithin buffer 140.

As discussed above, data caching process 10 may identify one or morerejected Mode Select commands (namely Mode Select command 144).Accordingly, data caching process 10 may notify 306 the host associatedwith each of these rejected Mode Select commands (namely servercomputer/controller 134).

In the event that a particular LUN (e.g., LUN 0) is restarted (e.g. dueto a crash or maintenance), data caching process 10 may reset 308 thecontrol of this particular LUN to a non-controlled state. Resetting 308control of the LUN to a non-controlled state may include populating 310the control field within buffer 140 associated with LUN 0 with a 0 andpopulating 312 the GUID field within buffer 140 associated with LUN 0with one or more zeros.

Relinquishing Control of a LUN

Assume for illustrative purposes that after gaining control of LUN 0,server computer/controller 100 no longer wishes to/needs to control LUN0. Accordingly, server computer/controller 100 may generate a ModeSelect command (Mode Select command 142) that results in servercomputer/controller 100 relinquishing control of LUN 0. Specifically,this Mode Select command to relinquish control of LUN 0 may includecontrol information that identifies control as 0 and host identifierinformation that identifies server computer/controller 100 (the hostthat last had control of the LUN).

Upon receiving 350 Mode Select command 142 from servercomputer/controller 100, data caching process 10 may process 352 ModeSelect command 142 to determine if the control information includedwithin Mode Select command 142 signifies an intent by the host (i.e.,server computer/controller 100) to relinquish control of e.g., LUN 0. Asdiscussed above, such intent would be signified if the controlinformation included within Mode Select command 142 identifies controlas 0.

If data caching process 10 determines that the control informationincluded within Mode Select command 142 signifies an intent torelinquish control of LUN 0, data caching process 10 may process thehost identifier information included within Mode Select command 142 toconfirm 354 that the host identifier information matches LUN controlidentifier information currently stored within buffer 140 (which definesthe host that currently controls e.g., LUN 0).

As discussed above, when a particular host controls a LUN, appropriateinformation concerning the controlling host is written to the bufferassociated with the LUN (namely buffer 140). Accordingly, if the GUIDinformation within buffer 140 matches the host identifier informationincluded within Mode Select command 142, the host that generated ModeSelect command 142 (namely server computer/controller 100) has theauthority to relinquish control of LUN 0. Therefore, the controlinformation and/or the host identifier information included within ModeSelect command 142 may be written 356 to buffer 140 to effectuate therelinquishing of control of LUN 0.

Since (as discussed above) the control information included within ModeSelect command 142 is a 0, when data caching process 10 writes 356 thecontrol information to buffer 140, data caching process 10 may merelyneed to write 358 a zero to the control field within buffer 140.

If the host identifier information included within Mode Select command142 matches the LUN control identifier information currently storedwithin buffer 140 and, therefore, control will be relinquished by therequesting host, data caching process 10 may flush 360 the host cacheassociated with LUN 0. Accordingly, in the event that servercomputer/controller 100 successfully relinquishes control of LUN 0, datacaching process 10 may flush the portion of first cache system 126associated with LUN 0 due to server computer/controller 100 no longercontrolling LUN 0 and, over time, the data included within LUN 0 will beoverwritten and will no longer match the data included within firstcache portion 126.

In the event that server computer/controller 100 subsequently receives aread request concerning data stored within LUN 0 (which servercomputer/controller 100 no longer controls), server computer/controller100 may retrieve the data directly from LUN 0 and (as servercomputer/controller 100 no longer controls LUN 0) will not save a copyof the retrieved data within first cache portion 126.

In the event that the host identifier information included within ModeSelect command 142 does not match the LUN control identifier informationcurrently stored within buffer 140, the requesting host does not havethe authority to relinquish control of LUN 0. Accordingly, data cachingprocess 10 may reject 362 the Mode Select command and may notify therequesting host that their Mode Select command was rejected.

Reboot after Crash of Host

During the course of normal operations, a host (e.g. servercomputer/controller 100 and/or server computer/controller 134) maycrash. This may be done unintentionally (e.g., due to a softwaremalfunction) or may be done intentionally (e.g. to protect the integrityof the data stored within data array 130). For example, if a hostreceived a write request to write data to a LUN that it was notcontrolling and the host processed such a request, the resulting datastored within the LUN would be corrupt, as the data stored within theLUN would not correspond to the copy of the data that was locally-cachedby the host that is controlling the LUN. Accordingly, the host receivingsuch a write request may intentionally crash to avoid corrupting thedata stored within the LUN.

Assume for illustrative purposes that server computer/controller 134crashed and is in the process of coming back online. Accordingly, uponsensing 400 that a reboot sequence is executing on servercomputer/controller 134 (due to the occurrence of a crash event), datacaching process 10 may determine 402 at least one LUN that was beingcontrolled by the host (e.g. server computer/controller 134) prior tothe crash event, thus define at least one target LUN.

When determining 402 which LUNs were being controlled by the host thatis executing the reboot sequence, data caching process 10 may accessconfiguration file 146 (which may be stored within data array 130) thatdefines which hosts have control of which LUNs). Accordingly and inresponse to server computer/controller 134 executing the rebootsequence, data caching process 10 may simply access configuration file146 and look up server computer/controller 134 to determine which LUNsserver computer/controller 134 was controlling at the time of the crashevent.

Assume for illustrative purposes that server computer/controller 134 wasonly controlling a single LUN (namely LUN 1) at the time of the crashevent. Accordingly, data caching process 10 may prepare 404 a ModeSelect command (e.g., Mode Select command 144) for the target LUNs,which in this example is only LUN 1. As discussed above, Mode Selectcommand 144 may define control information and host identifierinformation concerning the host (e.g., server computer/controller 134)Accordingly and in this example, the control information would be a 1(as server computer/controller 134 wishes to regain control of LUN 1)and the host identifier information would identify servercomputer/controller 134.

Data caching process 10 may process 406 Mode Select command 144 todetermine if the control information and host identifier informationincluded within Mode Select command 144 matches control information andhost identifier information included within a buffer (e.g. buffer 148)associated with the target LUN (e.g., LUN 1). If the control informationand host identifier information included within Mode Select command 144matches the control information and host identifier information includedwithin buffer 148, data caching process 10 may grant 408 control of thetarget LUN (i.e., LUN 1) to the requesting host (namely servercomputer/controller 134).

As discussed above, when a particular host controls a LUN, appropriateinformation concerning the controlling host is written to the bufferassociated with the LUN (namely buffer 148). Accordingly, if the GUIDinformation within buffer 148 matches the host identifier informationincluded within Mode Select command 144, the host identified within ModeSelect command 144 (namely server computer/controller 134) may regaincontrol of LUN 1.

If the control information and host identifier information includedwithin Mode Select command 144 does not match the control informationand host identifier information included within buffer 148, data cachingprocess 10 may deny 410 the requesting host control of the target LUN(e.g., LUN 1) and may notify 412 the requesting host of the denial.

Obtaining Control of LUN after Death of Host

Assume for illustrative purposes that server computer/controller 134regains control of LUN 1. However, subsequent to regaining control,server computer/controller 134 suffers a system board failure and,therefore, for the purposes of this example is considered to be offline(i.e., dead). As server computer/controller 134 is dead, it cannotrelinquish control of LUN 1 in the manner described above. Further,assume for illustrative purposes that server computer/controller 100wishes to obtain control of LUN 1 (which is currently being controlledby server computer/controller 134, which is dead).

Accordingly, server computer/controller 100 may prepare a Mode Selectcommand (e.g., Mode Select command 142) to obtain control of LUN 1(i.e., to seize control away from “dead” server computer/controller134). When preparing Mode Select command 142, the control informationmay be defined as 0 and the host identifier information may identifyserver computer/controller 100.

Upon receiving 450, Mode Select command 142 from servercomputer/controller 100 concerning LUN 1 (which is currently beingcontrolled by server computer/controller 134), data caching process 10may process 452 Mode Select command 142 to determine if the controlinformation and host identifier information included within Mode Selectcommand 142 signifies an intent by e.g. server computer/controller 100to seize control of LUN 1 from server computer/controller 134.

When determining if there is an intent to seize control of LUN 1 fromserver computer/controller 134, data caching process 10 may determine ifMode Select command 142 includes control information set to 0 andincludes host identifier information that identifies a host other thanthe host that is currently controlling LUN 1. Since LUN 1 is currentlybeing controlled by server computer/controller 134 and the hostidentifier information included within Mode Select command 142identifies server computer/controller 100, data caching process 10 willdetermine that there is an intent to seize control of LUN 1 from servercomputer/controller 134.

Since the control information and host identifier information signifiesan intent to seize control of LUN 1 from server computer/controller 134,the control information and host identifier information included withinMode Select command 142 may be written 454 to buffer 148 (which isassociated with LUN 1). When writing 454 control information and hostidentifier information included within Mode Select command 142, datacaching process 10 may simply write 456 a zero to the control fieldincluded within buffer 148.

When a host controls a LUN, the host will typically “check in”periodically (e.g. every two minutes) to show that they are still“alive” and controlling the LUN (as opposed to being dead). These hostsmay “check in” using the above-described Mode Sense command to determinethe status of e.g., buffer 148.

Accordingly and in the event that a host is indeed alive, upon using theMode Sense command (which reads the content of the buffer associatedwith the LUN), the host will be able to see if control of the LUN wastaken away. And if control was taken away, data caching process 10 mayallow 458 the host in question to use a Mode Select command (in themanner described above) to regain control of the LUN. However, in theevent that a host is “dead” such inquiries (in the form of a Mode Sensecommand) and such control corrections (in the form of a Mode Selectcommand) will not be made on behalf of the “dead” host.

Accordingly and continuing with the above-stated example, once servercomputer/controller 100 issues Mode Select command 142 to seize controlof LUN 1 from server computer/controller 134, server computer/controller100 may rest until after the expiry of a defined verification period.The length of this verification period is longer than the frequency atwhich a host “checks in”. As discussed above and in this example, a hostwho is in control of a LUN “checks in” every two minutes. Accordingly, asuitable length for the above-described verification period may be sixminutes, which is long enough for (in this example) servercomputer/controller 134 to “check in” and make their existence be known.

After the expiration of this verification period, data caching process10 may read 458 (via a Mode Sense command) the content of buffer 148(i.e. the buffer associated with LUN 1). In the event that servercomputer/controller 134 is still viable (i.e., alive), servercomputer/controller 134 would have “checked in” during the verificationperiod, noticed the changes made to buffer 148 by servercomputer/controller 100, and issued a Mode Select command to correct thesame and reestablish control of LUN 1. Accordingly, if data cachingprocess 10 determines that buffer 148 was modified by servercomputer/controller 134 during the verification period, data cachingprocess 10 may deem server computer/controller 134 still viable (i.e.,alive) and may abort 462 the attempt by server computer/controller 100to seize control of LUN 1 from server computer/controller 134. If,however, data caching process 10 determines that buffer 148 was notmodified by server computer/controller 134 during the verificationperiod, data caching process 10 may deem server computer/controller 134not viable (i.e., dead) and may transfer 464 control of LUN 1 fromserver computer/controller 134 to server computer/controller 100.

As will be appreciated by one skilled in the art, the present disclosuremay be embodied as a method, system, or computer program product.Accordingly, the present disclosure may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,the present disclosure may take the form of a computer program producton a computer-usable storage medium having computer-usable program codeembodied in the medium.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present disclosure. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present disclosure has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the disclosure in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the disclosure. Theembodiment was chosen and described in order to best explain theprinciples of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various embodiments with various modifications as are suited to theparticular use contemplated.

A number of implementations have been described. Having thus describedthe disclosure of the present application in detail and by reference toembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of thedisclosure defined in the appended claims.

What is claimed is:
 1. A computer-implemented method comprising:receiving, by a processor, a Mode Select command concerning a logicalunit number (LUN) from a first host, wherein the Mode Select commanddefines control information and host identifier information concerningthe first host, and the LUN is currently being controlled by a secondhost; processing, by the processor, the Mode Select command to determineif the control information and host identifier information includedwithin the Mode Select command signifies an intent by the first host toseize control of the LUN from the second host; and if the controlinformation and host identifier information signifies an intent to seizecontrol of the LUN from the second host, writing, by the processor, thecontrol information and host identifier information included within theMode Select command to a buffer associated with the LUN, wherein thebuffer includes a control field and a globally unique identifier (GUID)field.
 2. The computer-implemented method of claim 1 wherein writing thecontrol information and host identifier information included within theMode Select command includes: writing, by the processor, a zero to thecontrol field within the buffer.
 3. The computer-implemented method ofclaim 1 further comprising: after the expiry of a defined verificationperiod, reading, by the processor, the buffer associated with the LUN todetermine if the second host is still viable.
 4. Thecomputer-implemented method of claim 3 further comprising: allowing, bythe processor, the second host to modify the buffer during theverification period to indicate viability.
 5. The computer-implementedmethod of claim 3 further comprising: if the second host is stillviable, aborting, by the processor, the attempt by the first host toseize control of the LUN from the second host.
 6. Thecomputer-implemented method of claim 3 further comprising: if the secondhost is not still viable, transferring, by the processor, control of theLUN from the second host to the first host.
 7. The computer-implementedmethod of claim 1 wherein the first host and the second host areapplication servers.
 8. A computer program product residing on anon-transitory computer readable medium having a plurality ofinstructions stored thereon which, when executed by a processor, causethe processor to perform operations comprising: receiving a Mode Selectcommand concerning a logical unit number (LUN) from a first host,wherein the Mode Select command defines control information and hostidentifier information concerning the first host, and the LUN iscurrently being controlled by a second host; processing the Mode Selectcommand to determine if the control information and host identifierinformation included within the Mode Select command signifies an intentby the first host to seize control of the LUN from the second host; andif the control information and host identifier information signifies anintent to seize control of the LUN from the second host, writing thecontrol information and host identifier information included within theMode Select command to a buffer associated with the LUN, wherein thebuffer includes a control field and a globally unique identifier (GUID)field.
 9. The computer program product of claim 8 wherein theinstructions for writing the control information and host identifierinformation included within the Mode Select command include instructionsfor: writing a zero to the control field within the buffer.
 10. Thecomputer program product of claim 8 further comprising instructions for:after the expiry of a defined verification period, reading the bufferassociated with the LUN to determine if the second host is still viable.11. The computer program product of claim 10 further comprisinginstructions for: allowing the second host to modify the buffer duringthe verification period to indicate viability.
 12. The computer programproduct of claim 10 further comprising instructions for: if the secondhost is still viable, aborting the attempt by the first host to seizecontrol of the LUN from the second host.
 13. The computer programproduct of claim 10 further comprising instructions for: if the secondhost is not still viable, transferring control of the LUN from thesecond host to the first host.
 14. The computer program product of claim8 wherein the first host and the second host are application servers.15. A computing system comprising: at least one processor and at leastone memory architecture coupled with the at least one processorconfigured to perform operations including: receiving a Mode Selectcommand concerning a logical unit number (LUN) from a first host,wherein the Mode Select command defines control information and hostidentifier information concerning the first host, and the LUN iscurrently being controlled by a second host; processing the Mode Selectcommand to determine if the control information and host identifierinformation included within the Mode Select command signifies an intentby the first host to seize control of the LUN from the second host; andif the control information and host identifier information signifies anintent to seize control of the LUN from the second host, writing thecontrol information and host identifier information included within theMode Select command to a buffer associated with the LUN, wherein thebuffer includes a control field and a globally unique identifier (GUID)field.
 16. The computing system of claim 15 wherein writing the controlinformation and host identifier information included within the ModeSelect command includes: writing a zero to the control field within thebuffer.
 17. The computing system of claim 15 further configured toperform operations comprising: after the expiry of a definedverification period, reading the buffer associated with the LUN todetermine if the second host is still viable.
 18. The computing systemof claim 17 further configured to perform operations comprising:allowing the second host to modify the buffer during the verificationperiod to indicate viability.
 19. The computing system of claim 17further configured to perform operations comprising: if the second hostis still viable, aborting the attempt by the first host to seize controlof the LUN from the second host.
 20. The computing system of claim 17further configured to perform operations comprising: if the second hostis not still viable, transferring control of the LUN from the secondhost to the first host.
 21. The computing system of claim 15 wherein thefirst host and the second host are application servers.